Are.na is a 12+ year old online community primarily for artists and designers. The developers have been able to keep the community high quality and fresh by consistently making tasteful choices—everything from not running ads to ignoring design trends and avoiding attention-jacking.
There have been many, many clones, and you'll find that they seldom last or stay interesting to their core audiences.
Their usage of Arial is a throwback to their roots (in early del.icio.us and websurfing culture), and works well for the intended purpose of allowing the website to take a back seat to the content. From my perspective, it feels both "cool"—irreverent, contextually aware—and functional, and as such I think it's both a great brand move and a great design move.
Now: imagine you're building a product over decades, and you're committed to using a font that's old, limited and owned by Microsoft. No monospace version. Limited character set. No modern features like variable weights. And someone comes along and is like: "I'd love to re-draw the font so that you can have modern features, a clear license that's tailored to your needs, and as a bonus it'll be a great story and we can write an article about the process for marketing."
"We re-drew Arial." Hilarious! And what's more, we have more degrees of freedom for future designs, and maybe the font looks just slightly better.
That's what happening here. It's not satire, just good fun and functionality.